Northern Mike

I'm not sure if anyone will have an answer for this, but I figured I would ask for ideas and or thoughts.

I'm looking into what speeds I will need to be able to cross water with my lx2e formula offroad build.
Using the formula from barefoot skiing (weight ÷ 10 + 20 = minimum speed) comes out a bit low in my opinion. With this formula, I'm looking at 18 to 20mph for my buggy (weighs 9lb and change with battery, body and sand paw wheels).

With my current gearing (13/51), I'm in theory able to hit about 33mph (need to measure with GPS).
Thus buggy doesn't get up and dance like my over powered Rustler did so I'm not yet confident it will be able to walk on water.

Thoughts on speed requirements?

zackS30

Snowmibiles can do it. They weigh like 700lbs and can go across small water holes seemingly easy. They also have pladdles on the front so that helps keep the front above the water @ speed

Maj_Overdrive

My Feedback: (11)

Snowmobiles can do it because of their great power to weight ratio and also because on water they use a cleated track that acts as a paddle. That cleated track provides a pretty efficient way to put the power down and keep it moving. The variable transmission also keeps the engine in the powerband. The forward momentum and angle of the track is actually what keeps them from sinking, not the front ski's. If the ski's are in or touching the water it's just slowing the sled down as they aren't even needed for turning.

I've seen some videos of rc cars going across water. You definitely have the tires but I think you're a bit short on power and more importantly speed. Yes you can achieve 33mph on land but you won't be able to put the power down to the water to maintain it and possibly not minimum speed either. A tire on land moves the vehicle forward a distance equal to the tires circumference. On water you're relying on just each paddle to move the vehicle forward. So the space between paddles is like the tire is "slipping" and not moving the vehicle forward. Paddle wheel also isn't the most efficient and needs to spin faster than the vehicle is moving. If I had to guess I'd say 33mph gearing would be the bare minimum to maintain 20mph actual speed meaning you're cutting it real close. I personally wouldn't take the chance unless I knew I had the power and gearing for 45-50mph on land. I'd think an 8lb buggy sinks pretty good

Northern Mike

For the most part, you are right about sleds.I've ran open water a few times (not on purpose).
That being said, HP to weight isn't what make sleds on water possible.
The first sled I had on water was an 1985 SRV 540cc. I don't remember the weight and HP of it, but saying 600lb with me on it and 50hp shouldn't be too far off. On a good day, it would do just over 100mph. The last sled I hit open water with was a 1996 Formula 3 600cc with about 115hp and about 750lb with me on it.
Compare that to my current buggy setup at say 9lb and 1.6hp and 33mph....

What makes sleds and water possible is the surface area of the track, shape and speed. You can accelerate on water with a sled, just don't go wide open throttle as soon as you hit the water or you'll drowned it.

I used barefoot skiing as it was the only formula I could find and the surface area to weight was a lot higher.

I think the key will be the surface area, weight and speed.

Maj_Overdrive

My Feedback: (11)

You're completely right about surface area and the weight on that area playing a role in keeping things afloat on water. But as you said speed is also a key component in keeping an object afloat that would normally sink because it has too much weight on it's surface area.

I could very well be wrong but achieving this speed depends on the efficiency of pushing against the water to provide forward momentum. A sled with a cleated track has a larger surface area than 4 wheels and it also has more paddles in constant contact with the water moving it forward. This is why you can slow down and speed up (to a degree) with a sled on water. I've seen a few videos of FOFF guys lift for a split second and get right back on it to change direction on water but they don't really break that momentum or wheel speed much. In other words wheel speed is more critical and has to be higher to maintain floatation. At least thats what I get out of it.

Ttowntoolman

I can drive my sled to the waters edge, ski's touching water, bag the throttle and skip or water walk almost indefinitely with some finesse. Horse power to weight 125 horse @ 500 pounds. If you bag it and hold it tight, it will wheelie to vertical, cavitate the track and sink like a stone. FYI

Have seen videos of traxxas velineon powered rigs crossing lakes with paddles, looked sweet.

Northern Mike

I've seen many of the same videos probably. Most are rustlers running 3s.
Assuming everything is stock except the battery and rear tires, the 3500kv motor is running the rustler at a theoretical top speed of 45mph.
Based on my testing with my snow rustler, I can say about 25-30mph is needed to make the rustler float on snow with sand paws. I did a run in a snow storm one time.in about a foot of fluffy snow, I could float the rustler at about half throttle with hardly anything but the paddle tracks visible. I was running 1300w on 4s.

The big difference I find between my rustler and my ofna is the rustler is too light and floaty. The ofna is planted and sure footed.
There is close to 5lb weight difference between the two.

I'll see how it reacts when I get the taller pinion gear in it. When I run the semi-locked center diff (I have 3 center diffs, the semi-locked is filled with bearing grease) with the sand paws or badlands, it will lift the front wheels on launch.

Ttowntoolman

Yea, probably should completely lock the center diff or put a spool from the dirt oval car in it. I have locked a center diff with hot glue before, worked alright. I would think a wider truggy based paddle tire would be a big help here also. Do they make truggy paddles?

What if it sinks? I kinda got a hunch that a rustler may somewhat float, I could be wrong. It is light and the tires may provide enough floatation to keep it from going to the bottom. The heavier ofna may sink completely if you have a mishap. Make sure you get a VID.

Maj_Overdrive

My Feedback: (11)

I didn't even consider the diffs, good call. In my opinion grease doesn't semi-lock a diff. It may provide enough tension to wheely off the line but if it can't carry it at all it's unloading. If the diff unloads on water you're going to lose momentum. If you're serious about running on water I'd lock the center diff somehow, silly putty, 500k fluid or whatever. Semi locked would be using 50k fluid as it will still unload, but it won't unload as soon as the front tires are in the air.

Northern Mike

I didn't post any pictures in this thread, but I am running 2.8 Proline Sand Paws on it. They are the same tires as I had on my rustler (actually, the rears on the ofna are the rustler's tires modified). The big difference between running these on the rustler and on the ofna is the tires now have less side wall and I used a heavy gorellia tape instead of cheap duck tape. They do not balloon at all now.

Northern Mike

I've been messing around with different grease in the center diff and you are right about unloading. Even filled and pressurized (filled right to the top when closing up), it will unload. If I roll or back the diff a few turns, it will lock up for long enough to wheelie a few feet depending on traction.
I have a few spare diffs (5 or 6) so I'll probably try a few different setups. I'm not a fan of rotating mass so that will play a factor in the locked diff setup. I know its not much mass, but spinning weight is bad weight.
Ideally a locked center, semi-locked front and a slightly heavier rear diff would be what I would run.


If/when I do try a run on water, it won't be deep. I won't run it unless I can prove the theory that it will work.
The lack of extra available throttle like I had on my rustler will probably be my hold back.

Foxy

The trick to this is gearing that tops out at the optimum speed so that you don't have to worry too much about throttle control once you're on the water, you can just pin it. Try it on a big puddle to start with.

By the way, the formula for barefoot skiing assumes an average sized pair of feet in contact with the surface of the water. The surface area of your tires combined does not even come close to that of a pair of adult feet, so I would change the +10 modifier in the formula to at least +25 and start experimenting from there. Maybe I'm overkill.

Northern Mike

I did find the numbers from the skiing formula low, but it was the closest I could find and figured the weight to surface area ratio shouldn't be too far off.
That being said, it like a 10-15mph extra margin.

Ttowntoolman

Oh yea, those tires should do the job! Nice.

Northern Mike

Well I'm hoping 35-40mph will be the sweet spot.
My new bearings and pinions should be in this week.
My calculated theoretical top speed is 46mph without timing advance. I'm going to try and run my rpm gauge on the wheels tonight to see how the theoretical top speed compares to the actual with the current 13/51 gearing.
Would prefer to real world test the speed but I'm having placement issues on the buggy for my GPS. This damn buggy has less available space then my 1/10 traxxas rustler.
For safety, I'll knock off 5% of the bench test speed for real world speed. The lose should be less then 5% but for safety....
Wish I had some snow to test on. Will test on some freshly groomed beach sand and see how the video and tracks in the sand come out.

Northern Mike

I did a quick bench test of the buggy with my RPM laser gun.
With a reflector on all 4 tires and one on the center diff, I pulled the following numbers (note some magin of error as holding the gun, car and remote is a little tough).
Gearing: 13/51
Battery: 4s 5000mah 30C

FL: 2584
FR: 2750
RL: 2726
RR: 2525
C Diff: 8372
Measured motor kV (listed at 1900kv): 2219kv

Average calculated car speed :40.9mph
5% real world lose : 36.8mph.

Based on the measurements, the wheel bearings expected to arrive any day are needed.